Proper centering of a semiconductor wafer is essential during wafer processing and transport in order to avoid costly errors such as nonuniform processing and/or wafer collision. During semiconductor device processing a wafer must be accurately centered on a wafer support platform in order to ensure the wafer will receive uniform processing across its entire face (e.g., uniformly deposited layers). Similarly, due to the rapid decrease in mechanical tolerencing associated with continually decreasing system footprint (i.e., the decrease of a fabrication system""s lateral dimensions), a wafer must be accurately centered on the transfer mechanism in order to avoid collisions during semiconductor device transport, and in order to reduce the probability of the wafer sliding off or being dropped by the transfer mechanism. In addition some processes are sensitive to crystal orientation and require wafer alignment prior to being transferred to the processing chamber. Accordingly numerous wafer alignment and/or centering devices (i.e., positioning devices) exist.
Conventional centering devices typically comprise a rotatable platform having three or four wafer lift pins that lift a wafer off the platform and shift horizontally to center the wafer. A wafer position sensor positioned a small distance above the platform transmits information regarding the location of the wafer""s center point to the wafer lift pins. In practice, because the wafer lift pins"" horizontal movement is limited, the wafer lift pins may need to make multiple lift and shift operations to achieve the required movement of the wafer to achieve accurate centering. The wafer edge location is checked after each lift and shift operation to verify wafer centering was achieved, or to initiate additional wafer movement.
Like conventional centering devices, conventional wafer alignment devices typically comprise a platform having a rotatable surface and a wafer position sensor positioned a small distance above the platform. The wafer position sensor identifies when a flat/notched region of the wafer is in a predetermined position and transmits this information to the rotatable platform to cease rotation.
Because a wafer must be centered before it can be aligned, many conventional devices include both alignment and centering capabilities. However, whether for centering and/or alignment, wafer transfer to, wafer positioning at, and wafer transfer from conventional positioning devices (i.e., the wafer transfer and positioning operation) follows the same sequence: 1) a wafer handler extracts a first wafer from a multi-slot wafer carrier and transports the first wafer to the positioning device; 2) the positioning device positions the first wafer; and 3) the wafer handler returns the positioned first wafer to the multi-slot wafer carrier. Thereafter the sequence repeats and the wafer handler extracts a second wafer from the multi-slot wafer carrier and transports the second wafer to the positioning device, etc.
As indicated by the sequence described above, conventional wafer positioning devices allow only the wafer handler or the positioning device to operate at a given time (i.e., serial operation); the wafer handler remains idle while the positioning device operates, and the positioning device remains idle while the wafer handler operates. Such serial operation wastes equipment operating time, reducing throughput and increasing wafer costs. Accordingly a need exists for an improved wafer positioning device.
The present invention provides an improved wafer positioning device having a wafer storage location in addition to a wafer platform and a wafer position sensor. The wafer storage location enables simultaneous operation of the wafer handler and the positioning device (i.e., parallel operation), thereby allowing the inventive positioning device to achieve significant throughput increases.
In a first aspect one or more wafer storage locations exist above the wafer position sensor and the wafer position sensor is mounted so as to allow a wafer to travel therepast (e.g., rotatably or retractably mounted). In a second aspect additional or alternative wafer storage locations exist between the wafer platform and the wafer position sensor. A wafer presence sensor detects the presence of a wafer in the region between the wafer platform and the wafer position sensor and alters the operation of the wafer positioning device accordingly.
In a first embodiment the storage location is formed by three rotatable towers spaced evenly about the wafer platform at a position just beyond the circumference of the wafer to be positioned. Each tower has one or more wafer support portions positioned at desired elevations above the wafer platform. As the towers rotate the wafer support portions are selectively rotated inwardly for wafer storage or outwardly for wafer passage. Preferably towers comprising more than one wafer support portion are comprised of concentric tubes that allow independent rotation of each of the tower""s wafer support portions. The wafer lift pins of the inventive positioning device can elevate a wafer to the elevation of each wafer support portion.
In operation, initially each wafer support portion is rotated outwardly (i.e., in the wafer passage position). The inventive positioner positions a first wafer, and the wafer lift pins elevate the positioned first wafer. Thereafter, the towers rotate to position the relevant wafer support portions under the wafer, creating a wafer storage location; while a wafer handler, operating in parallel with the inventive positioner, retrieves a second wafer from a wafer carrier, deposits the second wafer on the wafer platform and immediately picks up the first wafer from the storage location. The first wafer may then be returned to the wafer carrier or placed in a processing chamber. Thus the wafer storage location enables continuous parallel operation of the wafer handler and the wafer positioning device.
In a second embodiment of the invention the storage location is formed by wafer lift pins that elevate from the wafer platform to store a wafer above the platform. Like the rotatable towers of the first embodiment, the wafer lift pins of the second embodiment each have a wafer support portion capable of selectively assuming a wafer supporting position and a wafer passage position. In operation, initially each wafer support portion is rotated inwardly. The inventive positioner positions a first wafer, and the wafer lift pins elevate the positioned first wafer to a storage location. The wafer handler then removes the first wafer from the storage location formed by the wafer lift pins storage, the wafer support portions of the wafer lift pins assume the wafer passage position and the wafer lift pins lower into position beneath a second wafer positioned on the wafer platform.
The inventive wafer positioning device is twice as productive as conventional wafer positioning device as nearly a 100% productivity increase is realized prior to positioning of a third wafer. Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.